This invention relates to a semiconductor device and a method of producing the same and, in particular, to a semiconductor device having a nonvolatile memory using a phase change material and a method of producing the same.
A semiconductor memory for use in a semiconductor device is categorized into a volatile memory which loses memory information when power supply is turned off and a nonvolatile memory which retains memory information even when power supply is turned off. For example, the volatile memory is a DRAM (Dynamic Random Access Memory) or a SRAM (Static Random Access Memory) while the nonvolatile memory is an EEPROM (Electrically Erasable Programmable Read Only Memory) or a flash memory. In a recent mobile data terminal, for the purpose of miniaturization and power saving, use is often made of the flash memory which retains memory information even when power supply is turned off.
Recently, however, in order to achieve further miniaturization and power saving, attention is focused upon a phase change memory using a phase change material. The phase change memory is a nonvolatile memory in which two different crystal states, i.e., an amorphous state and a crystalline state, of the phase change material are used as memory information. Specifically, the phase change material is switched between the amorphous state having a high resistance value and the crystalline state having a low resistance value to represent memory information of “1” or “0”. As the above-mentioned phase change material, a chalcogenide material is used.
In the phase change memory, a rewriting operation is carried out in the following manner. The phase change material is supplied with a sufficient amount of Joule heat to be melted. Thereafter, the phase change material is rapidly cooled to be turned into an amorphous state having a high resistance. Alternatively, the phase change material is supplied with a smaller amount of Joule heat and then slowly cooled to be turned into a crystalline state having a low resistance. The amount of heat to be supplied and a cooling rate are controlled by an electric current value and a length (application time) of a pulse applied to the phase change material. Thus, the rewriting operation as the memory is carried out by switching the phase change material between the different crystal states to change a resistance value. A reading operation of the phase change memory is carried out by utilizing the fact that the value of a flowing electric current is different depending on the amorphous state or the crystalline state of the phase change material.
FIG. 1 shows a memory element structure of a related phase change memory. After a lower electrode 8 is formed, an interlayer insulation film 5 is deposited. Then, a contact hole is formed in the interlayer insulation film 5 and a heater electrode 1 is formed in the contact hole. On upper surfaces of the heater electrode 1 and the interlayer insulation film 5, a phase change film 2 and an upper electrode 3 are successively formed. The interlayer insulation film 5, for example, comprising an oxide film (SiO2) surrounds a side surface of the heater electrode 1 and forms a lower layer of the phase change film 2.
When an electric voltage is applied between the upper electrode 3 and the lower electrode 8, Joule heat is produced to switch a crystal state of the phase change film 2 above the heater electrode 1. By switching the crystal state of the phase change film 2, an electric resistance of the phase change film 2 is changed. An area of the phase change film 2 where the crystal state is switched is represented as a phase change region 4. In order to switch the crystal state of the phase change film 2, a temperature not lower than about 600° C. is required. However, by a limited amount of electric current, only a limited area of the phase change film 2 can be heated to a high temperature not lower than 600° C. Therefore, as illustrated in the figure, the phase change region 4 of the phase change film 2 is an area around a contact surface between the heater electrode 1 and the phase change film 2. However, because the interlayer insulation film 5 includes oxygen, the phase change film 2 in the phase change region 4 and the heater electrode 1 are oxidized at such a high temperature not lower than 600° C. so that the phase change memory is varied and degraded in rewriting characteristics.
The phase change memory is described in the following patent documents. Japanese Unexamined Patent Application Publication (JP-A) No. 2005-340837 (Patent Document 1) discloses a semiconductor device comprising a molding film having a protruding portion, a phase change film and an upper electrode formed on the protruding portion of the molding film, and a lower electrode connected to the phase change film. The phase change film and the upper electrode are covered with oxygen barrier films on upper and lateral sides thereof. The molding film comprises an insulation film having a high thermal conductivity and serving as an oxygen barrier, for example, a silicon oxynitride film or a silicon nitride film. Thus, the phase change film is covered with the oxygen barrier films so as to avoid penetration of oxygen and oxidization of the phase change film.
Japanese Unexamined Patent Application Publication (JP-A) No. 2006-19688 (Patent Document 2) discloses a phase change memory comprising a lower electrode, an oxide film formed on the lower electrode and provided with a contact hole, and a spacer, a phase change film, and an oxide film which are formed in the contact hole. With the above-mentioned structure, a contact area between the lower electrode and the phase change film is reduced so that the amount of a rewriting current is reduced. International Patent Publication WO 2003/85740 A1 (Patent Document 3) discloses a non-volatile memory comprising an organic dielectric film provided with a hole penetrating therethrough. On one side of the organic dielectric film, an inorganic dielectric film, a recording layer, and an upper electrode are formed. In the hole, a heater electrode is formed. On the other side of the organic dielectric film, a lower electrode is formed. A part of the organic dielectric film which would be heated to a high temperature in the vicinity of the recording layer is covered with the inorganic dielectric film so that the organic dielectric film is prevented from being deteriorated due to exposure to a high temperature.
As described above, upon rewriting the phase change memory, it is necessary to supply the heater electrode with an electric current to produce heat so that the phase change region is heated to a temperature not lower than 600° C. If the insulation film around the heater electrode or the phase change film is an oxidizing insulation film, for example, if the interlayer insulation film 5 is a silicon oxide film (SiO2), a heater electrode material or the phase change film is oxidized. If the heater electrode material is tungsten (W) or titanium nitride (TiN), the heater electrode material is oxidized into WOx or TiON. Consequently, a resistance value of the heater electrode is changed to cause variation or degradation of the rewriting characteristics of the phase change memory. In the following description, a film allowing permeation of oxygen, such as a silicon oxide film, or a film causing diffusion supply of oxygen is called an oxidizing insulation film. On the contrary, a film inhibiting permeation of oxygen or a film causing no diffusion supply of oxygen is called an anti-oxidizing insulation film.
In order to avoid the above-mentioned problem, Patent Document 1 discloses to cover the heater electrode or the phase change film with the anti-oxidizing insulation film. However, in Patent Document 1, the molding film as the interlayer insulation film is formed by a silicon oxynitride film or a silicon nitride film. Thereafter, on the side surface of the phase change film, a side wall comprising a silicon nitride film is formed. Further, these films are entirely covered with a silicon nitride film, a stress relaxation film, and a silicon nitride film. Thus, the heater electrode and the phase change film are entirely surrounded by the silicon nitride film so as to prevent the heater electrode and the phase change film from being oxidized.
However, when the heater electrode and the phase change film are entirely covered with the silicon nitride film, new problems will arise. For example, if the silicon nitride film is thick, a crack is caused in the insulation film due to a film stress. In order to relax the stress, the stress relaxation film is required. This results in an increase in number of steps. Further, since the silicon nitride film has a relative dielectric constant higher than that of the silicon oxide film, a wiring capacitance or line-to-line capacitance is increased. This results in degradation in circuit characteristics such as an increase in power consumption or decrease in operation frequency.